The present invention relates to a diode limiter device for reflecting and attenuating microwaves propagated in a waveguide.
For example, in a receiving circuit of a radar device, a diode limiter device is disposed at a waveguide portion located between the receiving circuit and an oscillator in order to reflect and attenuate microwaves so that the microwaves of large power pulse-oscillated by the oscillator can be prevented from being introduced and receiving circuit can be prevented from damaging.
For a conventional diode limiter device of this kind, a PIN diode is used. Because the PIN diode is destroyed when the power of microwaves handled increases and the pulse width increases, a separately excited bias system in which an SBD (Schottky-barrier diode) is combined with microwaves and the detection current obtained with the SBD is used is employed for the bias of the PIN diode to improve handling power (max. rating) capabilities.
FIG. 5 shows a constitution of a conventional diode limiter device, and FIG. 6 an equivalent circuit. In this configuration, SBD 53 is fixed to an inner wall surface of a waveguide passage 51a of a waveguide 51 by an SBD post 52 equipped with a microwave choke in such a manner to penetrate H surface (magnetic field surface) of the waveguide 51. When the SBD 53 is combined with microwaves inside the waveguide 51 and the detection current flows the diode, the current is converted into detection voltage by a resistor 55 for serving as both load resistor connected between a top end of the SBD post 52 and the waveguide 51 and recovery time adjuster. The SBD post 52 is fixed to the waveguide 51 via an insulator 54.
A PIN post 57 for supporting a PIN diode 56 penetrates a screw 58 equipped with a choke and is electrically connected to the top end of the SBD post 52. By this configuration, the detection voltage by the SBD 53 is applied to the PIN 56 as bias. The PIN post 57 and the screw 58 are insulated and separated via an insulator 59.
Under this constitution, rotation of the screw 58 adjusts a length of a coaxial line portion 60 composed of the PIN post 57 and the waveguide portion and is able to adjust passing characteristics of the diode limiter. Numeral 61 is a diode limiter portion on the second stage, and since the microwave power is greatly lowered in this portion, a self-bias system can be used for the PIN diode, and a separately exciting bias system by the SBD is not adopted.
FIG. 7 shows a constitution of another conventional diode limiter device and FIG. 8 an equivalent circuit. In this configuration, an end portion 73a of a PIN post 73 which supports a PIN diode 72 is formed to be thick, where a spot facing 73b is formed, and in the spot facing 73b SBD 74 and one end of the resistor 75 for serving as both load resistor and recovery time adjuster are soldered. The PIN post 73 is fixed in such a manner that it is pressed by an insulation cylinder 76 and a screw 77, and the SBD 74 and the other end of the resistor 75 are soldered with a penetration hole 77a provided at the center of the screw 77, to achieve continuity with the waveguide 71. Numeral 78 is a diode limiter portion at the second stage, which has a constitution similar to that described above.
However, because with the diode limiter device shown in FIG. 5 and FIG. 6, the constitution for arranging the SBD 53 becomes complicated, there is a problem that poor productivity and high cost result. Since the SBD 53 is located in front of the PIN diode 56, the waveguide 51 must be elongated.
Because with the diode limiter device shown in FIG. 7 and FIG. 8, the SBD 74 and the resistor 75 are arranged three-dimensionally, there is a problem that poor productivity and high cost result as in the case of the diode limiter device shown in FIG. 5 and FIG. 6. In addition, there is also a problem that the adjusting function for variations in electrostatic capacity of the PIN diode 72 is unable to be provided because the length of the coaxial line formed with the PIN post 73 and the waveguide 71 is fixed.
In view of the foregoing, it is an object of the invention to provide a diode limiter device which can achieve good productivity and low cost.